Add derivative to list of legal values for slope in gmetric help message.
Add derivative to list of legal values for slope in gmetric help message.

ganglia/monitor-core,torkelsson/monitor-core,fastly/monitor-core,phreakocious/monitor-core,NoodlesNZ/monitor-core,NoodlesNZ/monitor-core,phreakocious/monitor-core,ganglia/monitor-core,lawrencewu/monitor-core,sdgdsffdsfff/monitor-core,fastly/monitor-core,NoodlesNZ/monitor-core,sdgdsffdsfff/monitor-core,lawrencewu/monitor-core,ganglia/monitor-core,mjzhou/monitor-core,sdgdsffdsfff/monitor-core,NoodlesNZ/monitor-core,torkelsson/monitor-core,phreakocious/monitor-core,ganglia/monitor-core,lawrencewu/monitor-core,NoodlesNZ/monitor-core,ganglia/monitor-core,mjzhou/monitor-core,phreakocious/monitor-core,ganglia/monitor-core,phreakocious/monitor-core,lawrencewu/monitor-core,torkelsson/monitor-core,torkelsson/monitor-core,phreakocious/monitor-core,lawrencewu/monitor-core,ganglia/monitor-core,lawrencewu/monitor-core,sdgdsffdsfff/monitor-core,fastly/monitor-core,ganglia/monitor-core,NoodlesNZ/monitor-core,mjzhou/monitor-core,sdgdsffdsfff/monitor-core,lawrencewu/monitor-core,fastly/monitor-core,mjzhou/monitor-core,sdgdsffdsfff/monitor-core,torkelsson/monitor-core,torkelsson/monitor-core,fastly/monitor-core,mjzhou/monitor-core,torkelsson/monitor-core,fastly/monitor-core,NoodlesNZ/monitor-core,phreakocious/monitor-core,fastly/monitor-core,mjzhou/monitor-core,sdgdsffdsfff/monitor-core,mjzhou/monitor-core